Start-up circuit for providing a start-up voltage to an application circuit

ABSTRACT

A startup circuit for providing a startup voltage from a high voltage DC bus voltage to an application circuit comprising an integrated circuit package for at least a control circuit for driving at least one power switch of the application circuit; a dropping resistor in the integrated circuit package having a first terminal for coupling to the high voltage DC bus and for dropping the high voltage DC bus voltage to a reduced voltage level at a second terminal; further comprising a low voltage regulator in the package coupled to the second terminal for providing a startup regulated low voltage DC output at a preset level for powering at least one part of the application circuit during startup of the application circuit. The application circuit may be, for example, a switching mode power supply.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit and priority of U.S.Provisional Application No. 60/600,046 filed Aug. 9, 2004 and entitledSTART-UP RESISTOR FOR HIGH VOLTAGE SWITCHING REGULATOR, the entiredisclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a start-up circuit for providing astart-up voltage from a high voltage DC bus to a high voltageapplication circuit, for example, wherein the application circuit maycomprise a switching regulator circuit coupled to a high voltage DC bus.

In the past, start-up circuits comprising external resistors areprovided to application circuits wherein the application circuitcomprises an integrated circuit. It is desirable to be able to eliminatethe external resistor. However, this has been difficult to achieve inthe past due to the high voltage. Further, it is also desirable toprovide a start-up circuit which has an automatic shutoff feature whichshuts down the start-up circuit when the voltage regulator output issensed, i.e., when the voltage regulator output is generated by thevoltage regulator circuit. In this way, maximum power dissipation isconfined to a small time interval during start-up.

Further, it is desirable to be able to provide overcurrent protectionthat limits power dissipation in the start-up circuit in the event of anoutput short or overload condition.

As described previously, in the past, such start-up supplies wereprovided through an external resistor which increases cost and furtherwhich does not have the automatic shutoff capability nor overcurrentprotection.

The main purpose of this circuit is to supply a switching voltageregulator with power upon start-up from a high voltage DC bus before theregulator can self-supply. A typical DC bus voltage may be approximately600 volts, for example.

SUMMARY OF THE INVENTION

The invention provides a number of advantages. A dropping resistor isincorporated in the integrated circuit package for the controller of theapplication circuit, for example a switching regulator or the controland drive circuit of a switch regulator output circuit.

In addition, an automatic shutoff feature which shuts down the circuitwhen the voltage regulator output is provided. In this way, maximumpower dissipation is confined to a small time interval during start-up.

Furthermore, an overcurrent protection circuit that limits powerdissipation in the circuit in an output short or overload condition isprovided.

According to the invention, the above objects are achieved by a startupcircuit for providing a startup voltage from a high voltage DC busvoltage to an application circuit comprising an integrated circuitpackage for at least a control circuit for driving at least one powerswitch of the application circuit; a dropping resistor in the integratedcircuit package having a first terminal for coupling to the high voltageDC bus and for dropping the high voltage DC bus voltage to a reducedvoltage level at a second terminal; further comprising a low voltageregulator in the package coupled to the second terminal for providing astartup regulated low voltage DC output at a preset level for poweringat least one part of the application circuit during startup of theapplication circuit.

BRIEF DESCRIPTION OF THE DRAWING(S)

The invention will now be described in greater detail in the followingdetailed description with reference to the drawings in which FIG. 1shows the start-up circuit according to the present invention.

Other objects, features and advantages of the present invention will beapparent from the detailed description which follows.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Turning now to the drawings, FIG. 1 shows the start-up circuit accordingto the present invention. The start-up circuit comprises,illustratively, an NMOS start-up circuit showing the high and lowvoltage parts of the circuit. The high voltage section is containedwithin a high voltage well HW. The border of the high voltage well HWhas a polysilicon resistor (spiral R) which may be a spiral shapedpolyresistor which is used to shape the electric field. One terminal ofthe polyresistance is in the high voltage well and is connected to thehigh voltage DC bus VB. The other terminal is in the low voltage sideand is connected to the gate of the high voltage NMOS Q1. The value ofthe resistor spiral R is in the tens of megohms. It serves to turn onthe high voltage transistor Q1.

As shown in FIG. 1, the application circuit 10 is provided with voltagefrom the high voltage bus. The application circuit may be a switchingmode power supply or the control and drive portion for the output stage12 of a switching mode power supply (SMPS), shown illustratively as asynchronous buck converter output stage, although the SMPS can be anyother type of SMPS, such as a boost converter, back-boost, flybackforward converter, etc. According to the invention, the applicationcircuit 10 is typically contained within the square package (IC package)as the start-up circuit. Before the application circuit, which maycomprise a switching regulator, can self regulate to apply the outputvoltage V0, it is provided with the start-up voltage VCC by the start-upcircuit shown. Once the application circuit begins to supply theregulated output voltage, the high voltage start-up circuit is shut off.

The start-up circuit shown can also be used as a very low current powersupply X for circuits such as comparators, for example, drawing smallcurrents, for example, tens of microamps at about 12 volts. The VCCpoint is maintained at about 12 volts in the illustrated embodiment bythe current flowing in the branch with the zener diode, shownillustratively as two zener diodes Z1 and Z2. In the illustratedembodiment each zener diode has an avalanche voltage of 5 volts for atotal of 10 volts avalanche voltage. The voltage drop across thesezeners plus the voltage drop across resistor R3 add to make VCC. Thisgives about 10 volts plus the threshold of Q2 (about 1.5 volts) for VCC.

This start-up circuit regulates VCC through a feedback mechanism asfollows. If the voltage at VCC increases for some reason above itssteady state value, this causes the voltage across R3 to increasecausing Q2 to turn on more heavily, which then causes the gate nodevoltage of Q1 to decrease. This decrease in gate voltage decreases thedrain current of Q1 which then closes the feedback loop by causing adecrease in the voltage of VCC which initially increased. The oppositesituation results if the change in VCC is opposite, i.e., if VCCdecreases.

In the desired operation, the output is taken from the point labeledVCC. As mentioned above, the output voltage is about 12 volts in theillustrated embodiment. This voltage is only needed momentarily whilethe application circuit is starting up. Once the application starts upit is desirable to turn off the start-up circuit to save power. Aftersupplying and powering up the application circuit, for example, aswitched mode power supply, Q2 senses the power supply output at VCC andprovides the shutoff function of the circuit by sensing the voltageacross R3. The higher voltage at VCC and hence across R3 will turn on Q2more (VT approximately 1.5 volts) and will decrease the gate drive ofthe supplying transistor Q1. After the shutoff, the symmetrical deviceQ3 has its drain connected to ground and its gate/source to VCC. Thiscauses Q3 to turn on with the source and drain interchanged. Then, R1 inthe interchanged source circuit of Q3 limits the power dissipationduring the shutoff operation.

The circuit also provides short circuit protection. If VCC were to beshorted or overloaded, all that portion of the circuit below the VCCpoint would be eliminated and the source of Q1 would be connected toground through resistor R2, which may be, for example, 20 K ohms. Thiswill cause too much power dissipation from VB without a current limit.In this case, Q3 senses a greater voltage drop across R2 in the shortcircuit operation and turns on, bringing down the gate voltage of Q1,limiting the maximum current.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art.Therefore, the present invention should be limited not by the specificdisclosure herein, but only by the appended claims.

1. A startup circuit for providing a startup voltage from a high voltageDC bus voltage to an application circuit comprising: an integratedcircuit package for at least a control circuit for driving at least onepower switch of the application circuit; a dropping resistor in theintegrated circuit package having a first terminal for coupling to thehigh voltage DC bus and for dropping the high voltage DC bus voltage toa reduced voltage level at a second terminal; further comprising a lowvoltage regulator in the package coupled to the second terminal forproviding a startup regulated low voltage DC output at a preset levelfor powering at least one part of the application circuit during startupof the application circuit.
 2. The startup circuit of claim 1, whereinthe low voltage regulator comprises a first high voltage controlledswitch in series circuit with a first resistance, the series circuitcoupled between said first terminal and said low voltage DC output; asecond switch coupled between said second terminal and a referencepotential and having a control electrode coupled to a voltage sensorsensing said low voltage DC output; a voltage reference source forsetting the low voltage DC output; said voltage reference and voltagesensor being coupled in series between said low voltage DC output andsaid reference potential; whereby when said low voltage DC outputchanges, conduction through said second switch changes, thereby changingthe voltage at the control electrode of the first high voltagecontrolled switch to drive the low voltage DC output back to the presetlevel.
 3. The startup circuit of claim 2, wherein if the low voltage DCoutput increases, a voltage across said voltage sensor increases,causing said second switch to increase its conduction, thereby reducingthe voltage at the control electrode of said first high voltage switch,thereby decreasing the current in said high voltage switch and reducingthe low voltage DC output to the preset level.
 4. The startup circuit ofclaim 3, wherein if the low voltage DC output decreases, a voltageacross said voltage sensor decreases, causing said second switch todecrease its conduction, thereby increasing the voltage at the controlelectrode of said first high voltage switch, thereby increasing thecurrent in said high voltage switch and increasing the low voltage DCoutput to the preset level.
 5. The startup circuit of claim 2, whereinsaid second switch senses said low voltage DC output via said voltagesensor, so that, when a higher voltage at said low voltage DC outputsupplied by the switching regulator circuit is present after startup,said second switch will increase in conduction to decrease the voltageat the control electrode of the first high voltage switch such that thefirst high voltage switch turns off.
 6. The startup circuit of claim 5further comprising a third controlled switch in series with a secondseries resistance coupled between said second terminal and said lowvoltage DC output for limiting power dissipation in said second switchwhen said first high voltage switch is turned off.
 7. The startupcircuit of claim 6, wherein said third switch has a control electrodecoupled to said low voltage DC output through said first resistancewhereby, when said second switch turns on when the output of theswitching regulator circuit is present after startup on the low voltageDC output, said third switch turns on so that current flows through saidsecond series resistance.
 8. The startup circuit of claim 7, whereinsaid third switch comprises a symmetrical MOSFET, whereby, when saidthird switch turns on, the source and drain of the third switchinterchange.
 9. The startup circuit of claim 6, wherein said thirdswitch senses a voltage across said first resistance, whereby, if saidvoltage across said first resistance indicates a short circuit conditionat said low voltage DC output, said third switch operates to limit thecurrent in said first switch to a preset safe level.
 10. The startupcircuit of claim 9, wherein, if the voltage across said first resistanceindicates a short circuit condition at said low voltage DC output, saidthird switch turns on, reducing the voltage at said control electrode ofsaid first switch, thereby limiting the current in said first switch toa preset safe level.
 11. The startup circuit of claim 1, wherein saiddropping resistor comprises a polysilicon resistor contained in a highvoltage well.
 12. The startup circuit of claim 11, wherein thepolysilicon resistor comprises a spiral-shaped polysilicon resistordisposed at a border of the high voltage well for shaping the electricfield.
 13. The startup circuit of claim 12, wherein said first highvoltage switch is contained in the high voltage well.
 14. The startupcircuit of claim 13, wherein the second and third switches are containedin a low voltage well.
 15. The startup circuit of claim 2, wherein thevoltage reference source comprises at least one zener diode.
 16. Thestartup circuit of claim 6, wherein said first, second and thirdswitches comprise MOFSETs.
 17. The startup circuit of claim 2, furthercomprising a transient protection circuit coupled to the controlelectrode of said first switch.
 18. The startup circuit of claim 11,wherein said polysilicon resistor has a resistance in the tens ofmegohms.
 19. The startup circuit of claim 1, wherein the applicationcircuit comprises a high voltage application circuit.
 20. The startupcircuit of claim 19, wherein the high voltage application circuitcomprises a switching regulator.